This invention relates to a method for changing the sulfide chain distribution of a sulfide chain-bearing organosilicon compound having the following general formula (1):
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Smxe2x80x94R3xe2x80x94Si(OR1)(3xe2x88x92p)(R2)pxe2x80x83xe2x80x83(1)
wherein R1 and R2 each are a monovalent hydrocarbon group having 1 to 4 carbon atoms, R3 is a divalent hydrocarbon group having 1 to 10 carbon atoms, m has an average value of 2 less than mxe2x89xa66, and p is 0, 1 or 2. More particularly, it relates to a method for converting a sulfide chain-bearing organosilicon compound having the formula (1) to a sulfide chain-bearing organosilicon compound having a shorter sulfide chain.
In silica-loaded tires, bis-triethoxysilyltetrasulfide is widely used as a coupling agent between rubber and silica. However, when mixed with rubber and silica at elevated temperatures, this compound acts to increase the viscosity of the blend, which is inconvenient to subsequent operation.
To overcome this problem, shorter chain polysulfide compounds such as bis-triethoxysilylpropyldisulfide were proposed. For example, U.S. Pat. No. 5,663,395 discloses a method for preparing disulfide silanes using NaCN. This method, however, has the problem of using the toxic compound. It would be desirable to have a substitute safe method of preparing short sulfide chain-bearing organosilicon compounds at low cost.
The inventors proposed in U.S. Pat. No. 6,015,870 a method of preparing a short sulfide chain-bearing organosilicon compound by reacting a polysulfide silane of the general formula:
(RO)3SiC3H6SxC3H6Si(OR)3
wherein R is methyl or ethyl, and x is a positive number of 3 to 6 on the average, at least one dry sulfur compound: M12S or M2S wherein M1 is an alkali metal or ammonium and M2 is an alkaline earth metal or zinc, and a halogenoalkoxysilane of the general formula:
XC3H6Si(OR)3
wherein X is halogen and R is methyl or ethyl. When the short sulfide chain-bearing organosilicon compound is prepared by this method, however, there can also be produced a monosulfide chain-bearing organosilicon compound, that is, an organosilicon compound having a sulfide chain which does not fully participate in the reactions with silica and rubber.
An object of the present invention is to provide a safe and economical method for preparing, from a sulfide chain-bearing organosilicon compound of the formula (1), a sulfide chain-bearing organosilicon compound having a shorter average sulfide chain and having a minimal content of monosulfide chain-bearing organosilicon compound in its composition.
The present invention provides a method for preparing a sulfide chain-bearing organosilicon compound having the following general formula (3):
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Snxe2x80x94R3xe2x80x94Si(OR1)(3xe2x88x92p)(R2)pxe2x80x83xe2x80x83(3)
wherein R1 and R2 each are a monovalent hydrocarbon group having 1 to 4 carbon atoms, R3 is a divalent hydrocarbon group having 1 to 10 carbon atoms, n has an average value of 2xe2x89xa6n less than 6, and p is 0, 1 or 2. The method involves the step of reacting a sulfide chain-bearing organosilicon compound having the following general formula (1):
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Smxe2x80x94R3xe2x80x94Si(OR1)(3xe2x88x92p)(R2)pxe2x80x83xe2x80x83(1)
wherein R1, R2, R3 and p are as defined above, and m has an average value of 2 less than mxe2x89xa66 and m greater than n, an alkali metal or alkaline earth metal, a halogenoalkyl group-bearing organosilicon compound having the following general formula (2):
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Xxe2x80x83xe2x80x83(2)
wherein R1, R2, R3 and p are as defined above, and X is a halogen atom, and optionally, sulfur. With this method, the desired sulfide chain-bearing organosilicon compound, and especially a sulfide chain-bearing organosilicon compound of formula (3) wherein n has an average value of 2 to 3 and having a minimal content of monosulfide chain-bearing organosilicon compound in its composition is produced in high yields and at low cost.
Briefly stated, according to the invention, a sulfide chain-bearing organosilicon compound of formula (3) is obtained by reacting a sulfide chain-bearing organosilicon compound of formula (1), an alkali metal or alkaline earth metal, a halogenoalkyl group-bearing organosilicon compound of formula (2), and optionally, sulfur.
One starting reactant is a sulfide chain-bearing organosilicon compound having the following general formula (1).
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Smxe2x80x94R3xe2x80x94Si(OR1)(3xe2x88x92p)(R2)pxe2x80x83xe2x80x83(1)
In the formula, R1 and R2 are independently selected from monovalent hydrocarbon groups having 1 to 4 carbon atoms, for example, alkyl and alkenyl groups such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, allyl and methallyl. R3 is selected from divalent hydrocarbon groups having 1 to 10 carbon atoms, for example, alkylene, arylene and alkenylene groups and combinations thereof, such as methylene, ethylene, propylene, n-butylene, i-butylene, hexylene, decylene, phenylene, and methylphenylethylene. The subscript m has an average value of 2 less than mxe2x89xa66 and preferably 3xe2x89xa6mxe2x89xa64, and p is equal to 0, 1 or 2.
Typical examples of the compound of formula (1) are given below.
(CH3O)3Sixe2x80x94(CH2)3xe2x80x94S4xe2x80x94(CH2)3xe2x80x94Si(OCH3)3
(CH3CH2O)3Sixe2x80x94(CH2)3xe2x80x94S4xe2x80x94(CH2)3xe2x80x94Si(OCH2CH3)3
In the above-described compound, S has a distribution partially because of disproportionation reaction, so that its number is described as an average value. In formula (1), m has an average value of 2 less than mxe2x89xa66 and preferably 3 less than mxe2x89xa64.
Any alkali metal or alkaline earth metal may be used herein although metallic sodium or potassium is preferred.
The halogenoalkyl group-bearing organosilicon compound used herein has the following general formula (2).
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Xxe2x80x83xe2x80x83(2)
In the formula, R1, R2, R3 and p are as defined above, and X is a halogen atom such as Cl, Br or I.
Typical examples of the compound of formula (2) are given below.
(CH3O)3Sixe2x80x94(CH2)3xe2x80x94Cl
(CH3O)3Sixe2x80x94(CH2)3xe2x80x94Br
(CH3CH2O)3Sixe2x80x94(CH2)3xe2x80x94Cl
(CH3CH2O)3Sixe2x80x94(CH2)3xe2x80x94Br
(CH3CH2O)3Sixe2x80x94CH2CH(CH3)CH2xe2x80x94Cl
When the reaction is conducted, sulfur is added if desired for adjusting the length of sulfide chain. It is not preferred to add a large amount of sulfur because the resulting product contains more monosulfide chain-bearing organosilicon compound.
The use of a solvent is optional when the end compound is prepared according to the invention. A solventless system is acceptable. Examples of the solvent, if used, include aliphatic hydrocarbons such as pentane, hexane, heptane and octane, aromatic hydrocarbons such as benzene, toluene and xylene, alcohols such as methanol and ethanol, and ethers such as dibutyl ether, tetrahydrofuran and dioxane. Of these, the ethers such as dibutyl ether, tetrahydrofuran and dioxane and the alcohols such as methanol and ethanol are preferred.
The method of the invention may be carried out in any desired order. For example, once the compound of formula (1) is reacted with the alkali metal or alkaline earth metal and optionally, sulfur, the reaction product is reacted with the compound of formula (2). Alternatively, a reactor is charged with the compound of formula (1), the compound of formula (2) and optionally, sulfur, and the alkali metal or alkaline earth metal is added thereto for reaction.
The reaction temperature is not critical and generally ranges from room temperature to about 200xc2x0 C., and preferably from about 60xc2x0 C. to about 170xc2x0 C. The reaction time is usually about 30 minutes or more. The reaction proceeds to completion within about 2 hours to about 15 hours.
For the reaction, the molar ratio of the respective reactants is preferably determined as follows. With respect to the molar ratio of the compound of formula (1) to the alkali or alkaline earth metal, the alkali or alkaline earth metal may be added in accordance with the desired value of n in formula (3). When the alkali metal is used, an equimolar amount of the compound of formula (2) is generally added. When the alkaline earth metal is used, a 2-fold molar amount of the compound of formula (2) is added. It is noted that the system becomes alkaline as the moles of the compound of formula (2) decreases, and becomes nearly neutral as the moles of the compound of formula (2) increases. More illustratively, when reaction is made among 1 mole of the compound of formula (1) wherein m has an average value of 4, 2 moles of the alkali metal and 2 moles of the compound of formula (2), there is obtained a compound of formula (3) wherein n has an average value of 2. The amount of sulfur added herein is arbitrary. For example, 0.5 mole of sulfur is added when it is desired for the average value of n to be 2.5 under the above-described molar ratios of the reactants. The amount of the solvent used is arbitrary. When the solvent is used, it may be distilled off in vacuum at the end of reaction and before or after the salt formed is separated by filtration.
The thus obtained compound has the following general formula (3).
(R1O)(3xe2x88x92p)(R2)pSixe2x80x94R3xe2x80x94Snxe2x80x94R3xe2x80x94Si(OR1)(3xe2x88x92p)(R2)pxe2x80x83xe2x80x83(3)
In the formula, R1 and R2 each are a monovalent hydrocarbon group having 1 to 4 carbon atoms, and R3 is a divalent hydrocarbon group having 1 to 10 carbon atoms, examples of which are as illustrated in conjunction with formula (1). The subscript p is 0, 1 or 2. The subscript n has an average value of 2xe2x89xa6n less than 6, and is smaller than m in formula (1), i.e., m greater than n, and preferably has an average value of 2xe2x89xa6nxe2x89xa63. In the compound (mixture) obtained by the inventive method, the content of the compound of formula (3) wherein n=1 is at most 5 mol %, and especially at most 2 mol %.
Typical examples of the compound of formula (3) are given below.
(CH3O)3Sixe2x80x94(CH2)3xe2x80x94S2xe2x80x94(CH2)3xe2x80x94Si(OCH3)3
(CH3O)3Sixe2x80x94(CH2)3xe2x80x94S3xe2x80x94(CH2)3xe2x80x94Si(OCH3)3
(CH3CH2O)3Sixe2x80x94(CH2)3xe2x80x94S2xe2x80x94(CH2)3xe2x80x94Si(OCH2CH3)3
(CH3CH2O)3Sixe2x80x94(CH2)3xe2x80x94S3xe2x80x94(CH2)3xe2x80x94Si(OCH2CH3)3
(CH3CH2O)3Sixe2x80x94CH2CH(CH3)CH2xe2x80x94S3xe2x80x94CH2CH(CH3)CH2xe2x80x94Si(OCH2CH3)3